WALK THE LINE
The rich chocolate tresses that Witherspoon wore in 2004 during filming for Walk the Line are in. "Deeper, bolder color is a trend for the season," says Goddard-Clark.

But if you're a natural blond like Witherspoon, Goddard-Clark doesn't recommend going the permanent route—or you could have difficulty getting back to your original pale hue. "When I first met Reese, it was right before she won an Academy Award for Walk the Line and her hair was in this ruddy state of color," says Goddard-Clark. "Different colorists had tried to make her blond again without success."

"Imagine hair as fabric and color as a stain," she adds. "Brown is a stain that doesn't want to come out—it will fade to a certain point through washings and normal wear, but it's particularly difficult to eradicate completely."

Goddard-Clark says using semipermanent color that's ammonia- and peroxide-free is ideal because it won't leave that unwanted residue and is easier to work with. One of her favorites: Clairol Loving Care. "It's like a watercolor instead of an oil painting—it's not as heavy of a pigment deposit, so the amount of coverage is a bit lighter. You can do more than one application to build your color to the desired depth."

Semipermanent products will eventually wash out, but if you start longing for lighter locks sooner than expected, Goddard-Clark offers a quick-fix solution: a cup of baby shampoo and two tablespoons of baking soda. Unlike adult shampoo, baby shampoo is more alkaline than acidic, boasting a higher pH level so as not to sting an infant's eyes. "It's enough to take color out on its own," says Goddard-Clark, but add a physical exfoliant like baking soda to the mix and you've got an even more powerful color-stripping formula.

HOW DO YOU KNOW
Witherspoon's creamy-dreamy locks might be her trademark, but Goddard-Clark varies the blond hue ever so slightly, depending on Witherspoon's movie of the moment. "A girl-next-door character might have more of Reese's natural dark-blond peeking through," she explains. When prepping the actress's strands for How Do You Know in 2009, however, Goddard-Clark went for "pale, buttery" highlights. The film, which premieres on December 17, features Witherspoon as a softball player, so Goddard-Clark wanted the hair color to have an outdoorsy, sun-drenched look.

Those with naturally dark-blond hair like Witherspoon's can achieve similarly lightened pieces with a home highlighting kit, says Goddard-Clark ("You can use a small toothbrush to highlight through your hair and blend away any unwanted deposits with olive oil"). If your tresses tend toward the browner end of the spectrum, though, Goddard-Clark recommends getting your shade professionally lifted.

WATER FOR ELEPHANTS
This past summer, Goddard-Clark took Witherspoon's blond hue into overdrive: "The character she plays in Water for Elephants [in theaters April 2011] is a 1930s circus siren, so we did a glamorous one-process platinum blond. It was a full-on oil bleach like Jean Harlow's."

But if you're thinking of getting this treatment done during your next salon visit, think twice—Goddard-Clark says the look requires an experienced colorist and lots of care: "It's very tricky, making sure the hair doesn't break. And in order for your roots not to show, you'll have to redo it often—we bleached Reese's hair every three weeks for continuity in the film."

To protect her strands from split ends, Witherspoon used a daily leave-in conditioner after her regular one, plus a 10-minute mask every three to four days. She also kept locks undone as much as possible, per Goddard-Clark's suggestion.

"Plastic clips and rubber bands can easily tear bleached hair—especially when it's wet," says the mane maestro, who notes that hair in general is more fragile damp than dry because the hydrogen bonds connecting the proteins within are broken down by water. Need proof? Just think of those times you got caught in the rain and your precious flatironed hair reverted back to its naturally frizzy state. "Your hair is like a paper towel," says Goddard-Clark. "If you tug at it when it's dry, it's got a bit more give and strength. But once water or any sort of liquid is introduced, it molds and shreds more easily."

Keeping platinum hair away from heat is another necessary precaution, says Goddard-Clark: "When the women were doing those bleaches in the '20s and '30s, they weren't doing blow-dries; it was a shampoo set or finger waves. The cumulative damage on superblond hair from heating instruments can be intense." Because of this, Witherspoon's on-set stylist crafted her retro-looking curls without the use of hot tools.

Want to see what you'd look like as a platinum blond—without actually taking the plunge? Test out Witherspoon's Hollywood hue, courtesy of our virtual makeover tool!